1. Field of the Invention
The present invention relates to method and apparatus for removing dissolved contaminants from a solution. More particularly, the present invention provides method and apparatus for electrolytically and mechanically removing selenium from a solution.
2. Background of the Prior Art
A common problem today is the reclamation of chemical solutions so that they may be reused or safely discarded. Toxicity characteristic tests established by the United States government have placed severe limits on the amount of contaminants which can be included in waste solutions to avoid the undesirable and costly designation of "hazardous substance."
Among the more difficult of contaminants to remove from solution is selenium (Se). Government toxicity characteristic (TCLP) specifications require that selenium in waste streams must be 1.0 ppm or less. Although these restraints have generated a great deal of interest in finding a method for removing selenium from solution, present methods for attempting to accomplish selenium removal are all considered far more complicated and costly than should be necessary.
Most examples of existing selenium removal techniques center around various chemical procedures. U.S. Pat. No. 3,933,635 issued Jan. 20, 1976, to Marchant employs a metallic reducing agent to remove selenium. Similarly, U.S. Pat. No. 4,222,999 issued Sep. 16, 1980, to Weir et al. uses a procedure of chemical reduction of Se.sup.+6 to Se.sup.+4 and a chemical co-precipitation of Se.sup.+4 with ferrous iron or ferric oxide. U.S. Pat. No. 4,377,556 issued Mar. 22, 1983, to Hofirek also employs chemical co-precipitation, in this case with sulfur dioxide or sulphite solution. U.S. Pat. No. 4,910,010 issued Mar. 20, 1990, to Khalafalla employs bacterial induced bioreduction of Se.sup.+6 to Se.sup.+4, exchange reaction with pyrite (FeS.sub.2), and burning. U.S. Pat. No. 4,915,928 issued Apr. 10, 1990, to Marcantonio employs an strong-base anion exchange resin to remove selenide (S.sup.-2) from a wastewater effluent. U.S. Pat. No. 5,089,141 issued Feb. 18, 1992, to Murphy discusses using a chemical process for selectively removing organoselenium compounds and selenate (Se.sup.+4) from water supplies.
One of the difficulties recognized in a number of the above patents is that selenium tends to occur in multiple valance states, compounding the difficulty in removing all of it from any given solution. In its natural state, selenium tends to occur as elemental selenium (Se[0]), and, in non-organic natural water systems, as selenite (Se.sup.+4) and selenate (Se.sup.+6). Moreover, in addition to Se.sup.+4 and Se.sup.+6, in some industrial waste solution streams selenium is also found in the negative valence state of selenide (Se.sup.-2).
Accordingly, attempting to remove a substantial portion of dissolved selenium from a solution containing selenite, selenate, and selenide requires addressing both different positive valence states of Se.sup.+4 and Se.sup.+6 as well as the negative valence state of Se.sup.-2. Even with the on-going interest in the problem of selenium removal reflected in the above patents, applicants are aware of no instance where such a removal can be readily accomplished without performing multiple independent treatment procedures.
Although electrolytic cells have been recognized as one method of removing certain dissolved impurities from some solutions, the relatively complex nature of mixed positive and negative valence states of selenium dissolved in industrial solutions complicates its removal using such techniques. When initially confronted with this problem, applicants attempted to employ an electrolytic cell, similar to that disclosed in applicants' U.S. Pat. No. 5,009,868, to remove selenium in the cathode compartment of the cell. Unfortunately, this method removed only some of the dissolved selenium and was completely inadequate in reaching necessary purity levels.
Examples of previous attempts to remove chemical impurities through electrolysis are illustrated in a number of issued United States Patents, such as U.S. Pat. Nos.: 3,506,560 issued Apr. 14, 1970, to Grangaard; 3,954,594 issued May 4, 1976, to Recht; 4,564,432 issued Jan. 14, 1986, to Nagai et al.; 4,963,241 issued Oct. 16, 1990, to Brattan; and 5,006,216 issued Apr. 9, 1991, to Dietrich et al. Although these devices may work well for their stated purposes, none of these patents attempt to address the problem of removing various forms of selenium from solution or how to deal with mixed positive and negative valence states of a dissolved metal.
Accordingly, it is a primary object of the present invention to provide an improved method and apparatus for readily removing dissolved selenium from a solution.
It is a further object of the present invention to provide such a method and apparatus which can effectively remove all forms of dissolved selenium from solution, including selenite, selenate, and selenide, in a single easily performed process.
It is an additional object of the present invention to provide such a method and apparatus which can accomplish effective selenium removal while producing minimal secondary waste in the removal procedure.
It is another object of the present invention to provide such a method and apparatus which require minimal investment to implement and operate, including functioning with minimal supervision and maintenance.
It is yet another object of the present invention to provide such a method and apparatus which can rapidly and effectively treat relatively high volumes of solution at minimal cost.
These and other objects of the present invention will become evident from review of the following specification.